Light emitting devices are generally fabricated by the steps of forming a plurality of semiconductor layers on a semiconductor substrate to fabricate a multi-layer semiconductor substram having a pn junction, and fabricating the substrates to devices. Among these light emitting devices, GaP type light emitting diodes, which emit red light, are fabricated from a semiconductor substrate for the light emitting devices consisting of an n-type GaP single crystal substrate on which n-type and p-type GaP layers are formed.
The compound semiconductor GaP does not emit red light by itself even if a pn junction is formed in a substate consisting thereof, so that zinc (Zn) and oxygen (O) are doped into a p-type layer, and pairs of Zn--O, which become light emitting centers, are formed in the p-type layer. GaP type light emitting diodes fabricated from the semiconductor substate for GaP type light emitting devices emit red light having a peak wavelength of about 700 nm.
However, according to the conventional light emitting devices fabricated from the semiconductor substrate for GaP type light emitting devices processed by the above conventional method, there is a disadvantage in that substantially high luminance cannot be obtained.
The semiconductor device disclosed in the U.S. Pat. No. 3,951,699 to Naito et al. employs a single crystal substrate of gallium phosphide doped with Te, a n-type gallium phosphide layer on said gallium phosphide single crystal substrate, and a p-type gallium phosphide layer on the n-type gallium phosphide layer. In this red-emitting device, Naito et al. is concerned with increasing luminescence by varying the cooling rate and heat treatment. However, there is no disclosure in the Naito et al. patent of a semiconductor device having a single crystal substram of gallium phosphide, an n-type gallium phosphide layer thereon and a p-type gallium phosphide layer thereon, wherein the carbon concentration of the n-type gallium phosphide single crystal substram is more than 1.times.10.sup.16, but less than .times.10.sup.17 atoms per cc.
Japanese Patent 0016391 to Kawabata et al., discloses growing an n-type GaP epitaxial layer on an n-type GaP substrate using a Te melt donor impurity. The melt is doped with zinc as an acceptor impurity exceeding Te and the melt is slowly cooled to grow p layer. However, the Kawabata et al. patent does not disclose that in a semiconductor device formed from a n-type gallium phosphide single crystal substram with an n-type gallium phosphide layer thereon, and a p-type gallium phosphide layer on the n-type gallium phosphide layer, varying the carbon concentration in the n-type gallium phosphide single crystal substrate would increase or have any effect on the luminescence of the device.